Fiat 1996 Brava Service Manual page 214

Engine
Fuel system
Bravo-Btava ^
TO
10.
Air flow meter (Debimeter)
The air flow meter (Debimeter) is hot film type. In this system, a heated membrane is placed in a mea­
surement channel through which air is taken into the engine.
The film membrane is maintained at a constant temperature (~120°C above air temperature) by an adja­
cent heating coil.
The mass of air flowing through the measurement channel tends to remove heat from the membrane. To
keep the membrane at constant temperature, a certain current must therefore flow through the heating
coil. This current is measured by means of a Wheatstone bridge.
The measured current is therefore proportional to the flowing air mass.
1. Connector
2. Measuring channel
3. Hot film sensor
A= Air intake
NOTE This debimeter measures air mass directly (not volume) and thus eliminates problems of tem­
perature, altitude, pressure etc.
Operating description
The Wheatstone bridge (R3, R2, Rs, Rt+ R1 format) is in equilibrium when Rs is about 120°C more than
air temperature.
The air flowing through the membrane removes heat from Rs, and the bridge becomes imbalanced. This
sitaution is deteced by the circuit connected to IC1, which controls transistor T1 to an extent propor­
tional to bridge imbalance. T1 then sends more current through Rh to heat Rs and thus restore the
bridge to equilibrium.
Circuit IC2 measures the current flowing through Rh. This current maintains the bridge in equilibrium
and is thus proportional to the mass of air flowing through the air meter.
^
IC2
Usub
Uk(=)
R3Q
C4 R5
w—c=h
|R2
RH
RS
1RT
R1
IC1
"CTtr-tr
1 2 3 4
Wiring diagram of air flow meter
14 Publication no. 506.670/06